Mayank is trying to draw your attention to this:

https://docs.moodle.org/dev/Task_API

"code runs inside of a cron job" is unclear as to what you are trying to do. Here's an example of a task that runs on a schedule:

namespace mod_yourmod\task;
defined('MOODLE_INTERNAL') || die();

/**
* The scheduled task.
*
*/
class scheduled_clean extends \core\task\scheduled_task {
public function get_name() {
// Shown in admin screens.
return get_string('custom string', 'mod_yourmod'); // your visible to admins name of the task
}

/**
* Run the cleanup task
*/
public function execute() {
return someclass::do_a_task(); // call some other code/class to use the DB calls here.
}
}

HTH